I'm understanding this as you have 12V and ground at the vehicle harness (unplugged from pump's connector) for a few seconds when you turn the key on (priming). But, you don't get voltage when it's plugged into the pump's connector and turn the key on again to prime. If you unplug it and check for voltage again, it's there. Is this correct?
Do you get 12V and ground when cranking (still unplugged) ?
Screwy things happen with low voltage. After it sat for those few weeks, was the battery low when you first went to start it?
With the pump installed (any of them) and the tank in place, did you connect the ground on the filler neck to the body?